Balancing amplitude and phase

ABSTRACT

A system for balancing a signal having I and Q components includes means for cross correlating the I and Q components to produce a cross correlation product; means for adjusting the gain of each I and Q signal component in accordance with said cross correlation product; and means for adding one component with the adjustable gain of the other component to produce a phase-balanced signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/122,992,filed Apr. 15, 2002 which is a continuation of application Ser. No.09/233,791, filed Jan. 19, 1999 which application is incorporated hereinby reference.

BACKGROUND

1. Field of the Invention

The present invention relates generally to digital communicationtechniques. More specifically, the invention relates to a system andmethod for balancing the amplitude and phase of a received,quadrature-phase modulated signal.

2. Description of the Prior Art

One of the common methods for modulating digital signals is the use ofmultilevel systems or M-ary techniques. M-ary modulation techniques arenatural extensions of binary modulation techniques and apply to L-levelamplitude or phase shift keying. A commonly used quadriphase scheme iscalled quadrature phase shift keying or QPSK. Like all of the M-aryamplitude or phase schemes, its principle advantage is bandwidthreduction.

Since pulse rate f_(p) is:f _(p) =f _(s)log_(L) M,  Equation 1where f_(s) is the symbol rate and M is the number of messages; with Lrepresenting the number of modulation levels, the larger L is, thesmaller the pulse rate and hence, the smaller the bandwidth.

In telecommunication applications, QPSK modulates two different signalsinto the same bandwidth creating a two-dimensional signal space. This isaccomplished by creating a composite phase modulated signal using twocarriers of the same frequency but having a phase difference of 90degrees as shown in FIG. 1A. By convention, the cosine carrier is calledthe in-phase component I and the sine carrier is the quadraturecomponent Q. The I component is the real component of the signal and theQ component is the imaginary component of the signal. Each of the I andQ components are bi-phase modulated. A QPSK symbol consists of at leastone sample from both the in-phase I and quadrature Q signals. Thesymbols may represent a quantized version of an analog sample or digitaldata.

All phase modulated schemes must overcome the inevitable problem ofphase synchronization. For proper operation of QPSK signaling, the I andQ channels should have the same gain throughout processing both receivedchannels, keeping the I and Q channels uncorrelated. Mismatched signalgains or magnitudes between the uncorrelated I and Q channels createerrors when processing. Phase differences other than 90 degrees betweenthe signals cause spillover between the channels and similarly result indegraded performance.

Typical receivers exhibit different overall gains for the separate I andQ channels due to mismatched gains in the mixers, filters, and A/Dconverters caused by variations in component values due in part totemperature, manufacturing tolerances and other factors. Amplitude andphase imbalance between the I and Q channels result in the distortionsshown in FIGS. 1B and 1C, decreasing overall signal-to-noise ratio(SNR).

Prior art approaches taken to avoid amplitude and phase imbalance relyupon very precise circuitry controlling each gain stage with activetemperature compensation. These expensive designs require componentsthat are manufactured with extremely low temperature coefficients andwith the mixers for the I and Q channels custom matched duringmanufacture.

Accordingly, there exists a need for a system that balances theamplitude and phase of a QPSK signal upon reception increasing signalintegrity and thereby reducing bit error rate (BER).

SUMMARY

The present invention balances the amplitude and phase of a receivedQPSK signal that may have been corrupted during transmission. The outputfrom the system is a signal corrected in both amplitude and phase. Thesystem determines the amplitude of the I and Q channels of a receivedsignal, compares them, and applies a correction to one or both channelscorrecting amplitude imbalance. For phase imbalance, the systemcalculates the cross-correlation of the I and Q channels which shouldaverage to zero. A correction factor is derived from thecross-correlation product and is applied to both channels, returning thephase cross-correlation to zero.

Accordingly, it is an object of the invention to provide a system whichbalances the amplitude of a received QPSK signal.

It is a further object of the invention to provide a system whichbalances the phase of a received QPSK signal.

Other objects and advantages of the system and method will becomeapparent to those skilled in the art after reading the detaileddescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1A is a plot of a QPSK symbol, balanced in both amplitude andphase.

FIG. 1B is a plot of a QPSK symbol, amplitude imbalanced.

FIG. 1C is a plot of a QPSK symbol, phase imbalanced.

FIG. 2 is a block diagram of an amplitude balancing system in accordancewith the present invention.

FIG. 3 is a block diagram of a phase balancing system in accordance withthe present invention.

FIG. 4 is a vector representation showing phase correction.

FIG. 5 is a block diagram of a combined amplitude and phase balancingsystem in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred embodiment will be described with reference to the drawingfigures where like numerals represent like elements throughout.

An embodiment showing the amplitude balancing system 17 of the presentinvention is shown in FIG. 2 where two bi-phase modulated signals 19 areinput 21I, 21Q. Quantizing is the process of measuring the intensity ofa signal in each sample and assigning a digital number to that measuredvalue. Each time the sampling circuit samples the signal, it measuresthe intensity of the varying analog signal at that discrete moment intime. The input 23I, 23Q data streams represent the discrete samples ofdata assembled into finite words each having a plurality of bits. Thenumber of bits that define each word determine the total quantization ofeach sample or symbol. For example, six-bit quantization:quantization levels=2^(n)−1  Equation 2with n equal to 6 would yield a resolution of 63 levels. Desired signalresolution determines n.

Each signal 23I, 23Q component, I and Q, is coupled to an input of anamplifier 25I, 25Q which has an adjustable gain. The output 27I, 27Q ofthe amplifiers 25I, 25Q are coupled to an absolute value processor 29I,29Q to obtain the relative magnitudes of each incoming symbol 23I, 23Q.The output 31I, 31Q of the absolute value processors 29I, 29Q arecoupled to inputs of respective low pass filters 33I, 33Q.

The low pass filters 33I, 33Q time-average the received componentsymbols 23I, 23Q, giving additional weight to recent samples anddecreasing weight to previous samples. In the present embodiment 17, IIR(infinite impulse response) filters 33I, 33Q with one pole are used,however, other types of filters or different order IIR filters can alsobe used without deviating from the principle of the invention. The lowpass filter outputs 351, 35Q present averaged estimates of the sampleamplitudes output from the absolute value processors 29I, 29Q.

A summer 37 obtains the difference from the outputs 35I, 35Q of the lowpass filters 33I, 33Q producing an error reference signal 39. If the Iand Q components of an input signal 23I, 23Q are orthogonal to eachother, the error reference signal 39 will have zero magnitude,indicating a balanced symbol. If the error reference signal 39 producesa value other than zero, the symbols are not amplitude balanced.

A non-zero-value error reference signal 39 becomes an error correctionvalue. The reference signal 39 is coupled to an input of a hard limiterprocessor 41. The hard limiter 41 outputs a signal 43 smaller inmagnitude, either positive or negative, in dependence upon the errorreference signal 39. The hard limiter processor 41 clips the errorreference signal 39 magnitude thereby making the sign of the errorreference signal 39 a correction factor. This is done for simplifyingthe implementation, the hard limiter is not essential to the invention.

The output 43 of the hard limiter processor 41 is coupled to a leakyintegrator which is an accumulator 45. The accumulator 45 adds thepresent value input with an accumulated value from previous input valuesand outputs 47 a sum. Since the accumulator 45 has a finite bit width,over time, the accumulated value will self-limit in magnitude andplateau if errors persist and are great. The accumulated plurality oferror reference signals 39 in the internal accumulator of theaccumulator 45 will average to zero when the system reaches stasis.

The output 47 from the accumulator 45 is coupled to a gain input 49I,49Q on each adjustable gain amplifier 25I, 25Q. The amplifiers 25I, 25Qbalance the amplitudes of the received I and Q symbols 23I, 23Q,increasing or attenuating their gains in dependence with the accumulator45 output signal 47. As can be seen, the reference signal 39 is negativefeedback to the upstream amplification stages 25I, 25Q. A positivecontrol voltage at the gain input 49I, 49Q indicates a gain increase forthat amplifier; a negative control voltage indicates attenuation.

If the amplitudes of the input signals 23I, 23Q are not balanced, thesystem will adjust the variable amplifiers 25I, 25Q (attenuating onecomponent while boosting the other) according to the accumulator 45output signal 47 until the I and Q symbol amplitudes are within apredetermined tolerance. If the symbol gains are equal, but vary betweenreceived symbols, the system 17 will not effect correction. A downstreamautomatic gain control (AGC)(not shown) equalizes the system output 51I,51Q for further signal processing (not shown).

An embodiment showing the phase correction system 61 of the presentinvention is shown in FIG. 3. Two bi-phase modulated signals 19 areinput 63I, 63Q into the system 61. The input 63I, 63Q data streams 65I,65Q for the I and Q symbols are coupled to a first input 67I, 67Q ofparallel summers 69I, 69Q. The output 71I, 71Q of each summer 69I, 69Qare the system output 73I, 73Q and feedback for the phase correctionsystem 61. Both feedback lines 71I, 71Q are coupled to a mixer 75 forcorrelation. The mixer 75 cross-correlated output signal 77 is coupledto an integrator 79. The integrator 79 time-averages thecross-correlation product 77. The integrator output is coupled to a hardlimiter processor 83. The hard decision processor 83 limits themagnitude of the integrated cross-correlation product. The hard decisionprocessor 83 output 85 retains sign. The hard limiter processor 83output 85 is coupled to an accumulator input 87. The hard decisionprocessor 83 reduces implementation complexity, one skilled in this artwould recognize that it is not essential.

As previously discussed, the function of an accumulator is toaccumulate, over-time, the present input value with previous inputs. Thesum is output as a correction signal.

The correction signal 89 is coupled to a first input 91I of a variablegain amplifier 93I coupling the Q input 65Q with the I input 63I. Thecorrection signal 89 also is coupled to a first input 91Q of a variablegain amplifier 93Q coupling the I symbol input 65I with the Q input 63Q.

The correction signal 89 adjusts both amplifiers 93I, 93Q increasing ordecreasing their gain. The amplifier outputs 95I, 95Q are coupled to asecond input 97I, 97Q of the input adders 69I, 69Q.

The phase correction is shown as a vector representation in FIG. 4. Theadders 69I, 69Q subtract the portion of Q component 63Q from the Icomponent 651;I=x−ry,  Equation 3−I=−x−ry,  Equation 4

-   -   where r        cross correlation,        and the portion of I component 63I from the Q component 65Q;        Q=y−xr,  Equation 5        −Q=−y−xr,  Equation 6    -   where r        cross correlation,        in order to remove the cross correlation contribution from each.        Once the parts of the signals that result in the cross        correlation are removed, the outputs 71I and 71Q of the adders        69I, 69Q become uncorrelated I, Q and orthogonal in signal        space.

An alternative embodiment combining both systems correcting amplitude 17and phase 61 imbalance is shown in FIG. 5. The system 101 is a simpleseries connection outputting 103I, 103Q a symbol corrected in bothamplitude and phase. Another combined embodiment where the amplitudebalancer 17 follows the phase balancer 61 is also possible.

While specific embodiments of the present invention have been shown anddescribed, many modifications and variations could be made by oneskilled in the art without departing from the spirit and scope of theinvention. The above description serves to illustrate and not limit theparticular form in any way.

1. An apparatus for balancing I and Q components of digital modulatedsignals, the apparatus comprising: a first I amplifier for amplifying aQ component of the signals in accordance with a correction factor; afirst Q amplifier for amplifying an I component of the signals inaccordance with the correction factor; an I adder for adding the Icomponent of the signals and an output of the first I amplifier forgenerating a phase balanced I component of the signals; a Q adder foradding the Q component and an output of the first Q amplifier forgenerating a phase balanced Q component of the signals; a correlator forcorrelating the outputs of the I adder and the Q adder; and a correctionfactor generator for generating a correction factor from the correlationresults.
 2. The apparatus of claim 1 further comprising an accumulatorfor time averaging the correlation results, whereby the correctionfactor is generated from the time averaged values.
 3. The apparatus ofclaim 1 further comprising: a second I amplifier for amplifying the Icomponent of the signals in accordance with a gain factor; a second Qamplifier for amplifying the Q component of the signals in accordancewith the gain factor; and a gain factor generator for generating thegain factor for the second I amplifier and the second Q amplifier fromthe outputs of the second I amplifier and the second Q amplifier,whereby the second I amplifier and the second Q amplifier outputamplitude balanced I and Q components of the signals, respectively. 4.The apparatus of claim 3 further comprising an absolute value generatorfor generating an absolute value of the outputs of the second Iamplifier and the second Q amplifier, whereby the gain factor generatorgenerates the gain factor from the absolute value of the outputs of thesecond I and Q amplifiers.
 5. The apparatus of claim 4 furthercomprising a low pass filter for time averaging the output of the secondI and Q amplifiers, whereby the gain factor is generated from the timeaveraged values.
 6. A method for balancing I and Q components of digitalmodulated signals, the method comprising: amplifying I and Q componentsof the signals in accordance with a correction factor; adding the Icomponent of the signals and the amplified Q component of the signals togenerate a phase balanced I component of the signals; adding the Qcomponent of the signals and the amplified I component of the signals togenerate a phase balanced Q component of the signals; correlating thephase balanced I and Q components of the signals; and generating acorrection factor from the correlation results.
 7. The method of claim 6further comprising: time averaging the correlation results, whereby thecorrection factor is generated from the time averaged values.
 8. Themethod of claim 6 further comprising: amplifying I and Q components ofthe signals in accordance with a gain factor to generate amplitudebalanced I and Q components of the signals, respectively; generating thegain factor by comparing the amplitude balanced I and Q components ofthe signals; and feeding back the gain factor for amplifying the I and Qcomponents of the signals.
 9. The method of claim 8 further comprising:generating absolute values of the amplified I and Q components, wherebythe gain factor is generated from the absolute values.
 10. The method ofclaim 9 further comprising: averaging the output of the amplified I andQ components of the signals, whereby the gain factor is generated fromthe time averaged values.